The present invention relates to diffractive optical methods and apparatuses for performing pupil remappings, including within wavefront sensing systems.
Several different types of wavefront sensors have been used effectively for adaptive optics systems. These include Shack-Hartmann, curvature and shearing interferometer sensors. Each of these sensors has advantages and may be customized for the particular system. For example, curvature sensors have been coupled to bimorph deformable mirrors to take advantage of sensing the same wavefront derivative that the mirror needs for control. Shearing systems have been applied to segmented adaptive optics for tip/tilt and phase comparison of adjacent segments. For Shack-Hartmann sensors, the customization takes the form of selecting the number and arrangement of subapertures to match the deformable mirror.
A Shack-Hartmann sensor has several basic components: a lenslet array, a detector and data acquisition and analysis apparatus. Until recently there were relatively few techniques available for fabricating the lenslet arrays. However, with the advent of binary optics technology, many of these barriers have been removed.
Binary optics technology is the application of semiconductor manufacturing methods to the fabrication of optics. A lens or lens array is laid out on a computer CAD program and transferred to a photo-mask using an e-beam (or other) writing process. A series of photo-masks are used, in conjunction with various etch steps, to build up the structures of interest. This fabrication technique can be used to make arrays of lenses with .about.1 .mu.m features in completely arbitrary patterns. Lenslet arrays are straightforward to make with these methods, and can be extremely high quality with no dead space between elements.
FIG. 1 presents the basic fabrication sequence for making a binary optic, which is one technique for fabricating a diffractive optic (others include E-beam direct write, laser direct write, laser ablation, grey scale masks, melted photoresist, and ink-jet melted photoresist). (In the specification and claims, binary optics and diffractive optics are used interchangeably.) The desired surface shape is broken up into a series of discrete phase levels, with the overall shape approximated by these levels. These optics are binary only in the sense that they are made from discrete levels. Typically 16 level systems are used to make precision optics. The phase levels are fabricated in a digital fashion using a number of photolithography and etch steps. The masks for the photolithography process are designed using a customized CAD (computer-aided design) program and commercial mask layout software.
The advantage of binary optics is that the optical fabrication is not limited to spheres and simple surfaces. With some processes, the total etch depth is limited to a few micrometers, and the minimum feature size to about 1 .mu.m, and the total optic size is limited by the amount of computer storage available in the mask production computer. The feature size does not become a problem unless the f# gets very low. For the high f# lenslet arrays usually specified for wavefront sensing, this is rarely a difficulty. Furthermore, the lenslet size is usually fairly close to the size of the detector, and thus large optics are not required. It should be pointed out that none of these limits are hard and fast. Large binary optics have been produced, as well as extremely fast optics. Feature sizes less than 1 .mu.m can be obtained using direct write e-beam or x-ray lithography.
Using binary optics, an optical system can be laid out as an array of lenses, which is convenient for Hartmann sensing, or in any other arbitrary arrangements. In fact, the arrangement of lenses does not have to be regular or continuous.
Because a Shack-Hartmann sensor uses an array of lenses to sample the incoming wavefront, binary optics can be applied to advantage. In fact, lenslet arrays were one of the first demonstrations of binary optics. For a parabolic lens, the total sagitus for a lenslet can be written: ##EQU1## where d is the lenslet diameter, n is the index of refraction and s is the sag. For a typical f# 100 lens, with 32 lenslets per cm, this corresponds to a total sagitus of 0.85 .mu.m. While this is more than a wave for light in the visible spectrum, it is straightforward to build a lens with the complete contour. With a 16 phase level fabrication sequence, the lens RMS (root mean square) roughness would be &lt;0.02 .mu.m. For visible light (0.5 .mu.m) this is better than .lambda./20 and has an efficiency of &gt;99%. Even much larger or faster lenses would have adequate performance using this technique. Alternatively, a Fresnel lens could be laid out. This would have the advantage of maximum efficiency at a particular wavelength, but would be limited to an extremely narrow band because of strong chromatic aberration.
This lenslet array is also designed to have zero dead space between lenses. The lenslets are placed accurately to a (typically) 0.1 .mu.m grid. Since the features are specified to 1 .mu.m accuracy, there is no space between lenses; hence the array has a 100% fill factor. For wavefront sensing this can be important because any light leakage will degrade the noise performance of the total system.
Because of the 100% fill factor, and the high efficiency attainable for lenses that will operate over a broad range of wavelengths, binary optics is ideally suited for wavefront sensing and other diffractive optics applications.
By taking advantage of the arbitrary nature of binary optics, lenslet arrays can be customized for the particular adaptive optic system. Since the design of the array is determined by the software layout program, there is no fabrication penalty for designing complicated or esoteric patterns or surface shapes. For example, an asphere is no harder to design and fabricate than a sphere. Similarly, the designer is not limited to a particular aperture shape, size or pattern. The optic array can be designed to map from the telescope or deformable mirror pupil to that of the detector. In some cases, intermediate optics can be eliminated or simplified.
The present invention recognizes that there are several areas where such remapping can be applied. Optical system apertures are usually round, while detectors are usually square. By remapping from a round to a square pupil using a binary optics approach, the detector fill factor can be improved. In addition, the spacing of actuators on a deformable mirror may be different in x and y directions. These can also be adjusted. Further, often the size of the telescope pupil image is not the same as that of the detector, and relay imaging optics must be used. This can also be accomplished in the design of a proper binary optic.
The present invention employs customized lenslet arrays for Shack-Hartmann sensing and of methodologies used in the design of same. Novel remappings are presented affecting image size, aperture shape, segment shape, spot position and subaperture function.